1. Field of the Invention
The present invention relates to a printed circuit board assembly manufacturing system and, more particularly, to a such manufacturing system that can simultaneously produce various kinds of printed circuit boards in one production line.
2. Description of the Related Art
The extraordinary growth in demand for consumer electronics products in recent years has resulted in a concomitant growth in demand for printed circuit board (PCB) assemblies used in such products. Rapid technological advances have also shortened the life cycles of electronics products, as consumer preferences adjust to the availability of new product features and improved quality. To compete in markets where consumers demand ever-increasing quality along with low prices, electronics manufacturers must reduce the production costs associated with PCB assembly production.
PCB assembly (PCBA) manufacture has tended to carry particularly high costs arising from two sources: set-up operations and production defects. Set-up of a production line for a different PCBA product entails refitting line equipment, retraining line operators, adjusting product transport equipment such as conveyors, and distributing parts for the new product to the various work stations. A change of the line set-up from one product to another therefore requires extensive expertise and substantial amounts of time. These operations have necessarily idled the entire production line for the duration of the retooling process. The shortened product life cycles that have become characteristic of the electronics industry only exacerbate the impact of set-up costs, because shorter production runs necessarily increase the per unit costs of production.
Line parallelism has been proposed as a way to mitigate the effect of set-up operations on overall productivity. For example, U.S. Pat. No. 4,719,694, issued Jan. 19, 1988 to Herberisch et al., discloses a system that divides a large number of PCB process steps into several groups of related operations performed in succession at several parallel cells of work stations. Each cell has a buffer area to store incoming units for which the cell is not ready or outgoing units for which a succeeding cell is not ready. Changing the production line over to a new product model can therefore proceed from cell to cell sequentially, rather than stopping the entire production line until every cell has completed its set-up for the new model.
This system substantially reduces down time costs, but its effectiveness is limited because it requires extensive coordination as the set-up process progresses through successive cells of the production line. Once a set-up process begins, it must propagate through the entire production line before the line can resume an optimal production rate. Moreover, the set-up process cannot easily be suspended or interrupted. The disclosed system also only rearranges a multiplicity of individual process steps, and it therefore does not address the issue of quality control.
A related approach is proposed in U.S. Pat. No. 5,170,554, issued Dec. 15, 1992 to Davis et al., which discloses a method of reducing set-up overhead by arranging a known production bottleneck into temporal cells. A temporal cell is defined in terms of a group of products, each of which is capable of being processed through the bottleneck using a single machine set-up. The method provides for careful planning of the duration and scope of these cells to allow a reduction in the number of set-up cycles and to overlap set-up cycles with production cycles.
This method shares some of the advantages of the '694 system discussed above. However, the approach of the '554 patent also demands careful coordination between production and set-up. Also like the system of the '694 patent, this system does not enhance quality control. It also does not consider potential gains from reconfiguring the physical arrangement of the production line.
An alternative approach uses multiple production lines to manufacture several different product models simultaneously. For example, U.S. Pat. No. 5,355,579, issued Oct. 18, 1994 to Miyasaka et al., discloses a production line system that comprises several individual lines arranged in parallel. Redundancy in the system is reduced by directing the output of the several parallel lines to a common packaging station. Each individual line in the system operates independently of the other lines to produce a particular product model, with the only interaction between lines occurring indirectly through the packaging station.
The substantial parallelism of the '579 patent's system reduces idle time for the overall system by decoupling the system into semi-independent sub-lines. This result comes, however, at the price of equipment duplication to implement the parallel production lines. More importantly, set-up for a production line generates similar overhead costs whether the line stands alone or in parallel with several other lines. Each of the disclosed system's sub-lines must be stopped to be changed over to a new product set-up, just as if that sub-line stood as an independent production line. The sharing a common packaging station at the end reduces these costs somewhat. However, the relative benefit of this measure declines as each of the individual lines grows in complexity and length.
None of the references mentioned above addresses the second major source of lost productivity in manufacturing systems for PCB assemblies: the incidence of production defects. These defects arise because the assemblies typically are structurally and functionally complex. The production system must therefore reliably assemble numerous different components and provide for a multitude of testing procedures to ensure proper operation of completed units. Faced with these system requirements, a traditional view of mass production has taught the division of the process into numerous specialized tasks, whereby the performance of each task (whether by a human operator or by automated equipment) can be designed to maximize throughput.
Extensive task specialization can cause its own problems, however, because it generally multiples the number of discrete operations in a fabrication process. This discretization can quickly erode the efficiency gains of task specialization because it increases the costs of adjusting the system to changed circumstances. Task discretization therefore tends to increase both the occurrence of production problems and the difficulty of quickly detecting those problems.
This problem arises whether the production line consists of fully automated process station or also employs human operators. For example, suppose that a process involves three or four operators working in serial relation. If a recurrent defect begins to arise in work units output from the first operator station, the problem can be quickly corrected even if it is not detected until the affected work units begin to arrive at the last station. If the system distributes the same process steps across 15 or 20 workers, in contrast, it may incorporate the defect into many more units before the defect is noticed. The effect can be even more pronounced with automated stations, where defect detection may be delayed by test and measurement limitations.
Two other mechanisms particularly affect production systems that employ human operators. First, the presence of a latent defect may be more difficult to detect in a highly discretized process. For example, a human operator carrying out a highly specialized task is more prone to boredom or inattentiveness, which can cause errors or prevent their prompt detection. Moreover, in a line with more process stations (whether operator controlled or fully automated), the nature and cause of a defect and its likely effects must be assessed for more stations.
In the case of human-operated stations, problem assessments and corresponding instructions must be quickly communicated to the station operators. This communication facilitates a coordinated response and is particularly important when the defect results from the combined work operations of several stations. In addition, similar problems may occur across several parallel sub-lines, and an adequate response to these situations requires an additional level of communication and coordination.
It follows that when a manufacturing system includes a large number of discrete work stations, problems are more difficult to detect rapidly and coordinated responses are more difficult to implement. Parallelism, as disclosed in the references cited above, reduces the overhead costs of line set-up, but it does not mitigate and may exacerbate the difficulty of responding to problems that arise during production.
We have therefore found that the electronics manufacturing arts have lacked a production arrangement that reduces overhead costs from both line set-up and production defects. Such an arrangement should minimize the need to reconfigure line equipment for production of a different PCBA product. It should also allow line elements not involved in a set-up operation to continue production, as well as allowing rapid detection and response to problems that arise in production. Preferably, such an arrangement would address these goals at all levels of the system design. Ideally, such a system would amplify its efficiency gains both by incorporating modem computerized process control technologies and by realizing the system responsiveness that human operators can provide.